A ResourceTransformer implementation that modifies links in a CSS
file to match the public URL paths that should be exposed to clients (e.g.
with an MD5 content-based hash inserted in the URL).

The implementation looks for links in CSS @import statements and
also inside CSS url() functions. All links are then passed through the
ResourceResolverChain and resolved relative to the location of the
containing CSS file. If successfully resolved, the link is modified, otherwise
the original link is preserved.